There are basically two methods for producing the non-spherical surface of a glass progressive power ophthalmic lens. According to one, the surface of the lens is simply ground and polished to the desired curvature, and examples of this method are shown in U.S. Pat. Nos. 2,869,422; 3,623,800; 3,853,499; 3,913,274 and 4,050,192. The second method consists of gravity sagging a lens blank having an originally polished spherical surface onto a forming block shaped to provide the polished surface with the desired progressive shape after sagging. This technique is mentioned in British Pat. No. 1,234,272.
Neither of these two methods is very satisfactory for a number of reasons. The grinding method, for example, requires the use of complex surfacing equipment. The gravity sagging technique, on the other hand, because of the irregular shape of progressive lenses, requires that the lens be softened to such an extent that it becomes highly susceptible to having its surface damaged. In addition, neither method provides enough flexibility to permit the manufacture of a lens design which can disregard manufacturing requirements and address only optical requirements. In other words, the present methods do not permit optimization of the lens design because of limitations placed on the design by manufacturing requirements.